The five paragraph essay is a favorite denizen of introductory English composition classes. The form is as follows:

Paragraph One:
Introduction.  The first paragraph tells us what the paper is going to say and, ideally, why we should care.  The final sentence in this paragraph is the paper's thesis.

Paragraphs Two-Four:

Supporting evidence.  These three paragraphs are where the actual arguing occurs and evidence is presented.  Especially quotes.  It seems like the five paragraph essay is made up of quotes like the human body is made up of water.

Paragraph Five:

Conclusion.  Usually the final paragraph is a brief restatement of what the essay has said.  The thesis, rephrased, can always be found lurking around here.  No new information should be presented in the fifth paragraph.

Despite its name, the five paragraph essay can contain any number of paragraphs.  As students progress, they are encouraged to include more evidentiary paragraphs while leaving the rest of the form alone.  As their arguments become more and more sophisticated, this form rapidly becomes unwieldy.

Sadly, most high-school composition classes never progress past the five paragraph essay.  This results in an unfortunately large number of college freshmen getting low marks on their first paper.